1. Field of Invention
The present invention generally relates to power supplies, and more particularly, to switched capacitor charge pump power supplies for generating a regulated output voltage different from the supply voltage.
2. Description of Related Art
An unregulated switched capacitor DC/DC converter includes a switch array. FIG. 1A illustrates an existing converter topology. This converter utilizes one flying capacitor and four switches. FIG. 1B illustrates another existing topology with two flying capacitors and nine switches.
FIG. 2 illustrates a three terminal device that is equivalent to the switch array. By connecting the TOP, MID and BOT terminals with input, output, and ground in various combinations, step-up, step-down and inverter topologies are created.
FIG. 3 illustrates a 2:1 step-down charge pump. Two-phase non-overlapping clocks are used to drive the switches. In phase 1, the charging phase, switches S1 and S3 are ON. Therefore, flying capacitor C1 is connected to an input supply voltage through the TOP terminal and charged. In phase 2, the pumping phase, flying capacitor C1 is connected to the output through the MID terminal. In this pumping phase the charge of flying capacitor C1 is transferred to an output capacitor Cout.
Charge pumps are regulated by various methods. In the method of hysteretic control a charge pump runs in a hysteretic mode. The hysteretic method can include pulse skipping, pulse frequency modulation, or the “bang-bang” process. The charge pump controls the output voltage into a voltage window. If the output reaches an upper threshold of the window, the oscillator of the circuitry is disabled and the power switches are turned OFF until the output voltage decreases to a value below a lower threshold. At this time the oscillator is enabled again and the switches are turned ON. This method can achieve high efficiency, especially in light load conditions. However, it can generate high current spikes and a large ripple in the output.
Another method to control charge pumps is called linear or analog control, or the Rdson modulation. A charge pump with a linear control operates at an essentially constant frequency. The charge pump is regulated through the analog, i.e. continuous modulation of the resistance of the switches, which are ON. The linear control method is capable of producing low noise.
FIG. 4 illustrates a charge pump with a linear control loop 1. Linear control loop 1 will be described in more detail later. In general, linear control loop 1 includes a charge pump 15 with TOP, MID, and BOT terminals, an output terminal 27 being coupled to the MID terminal. Linear control loop further includes a resistive voltage divider 49, connected to output terminal 27. A feedback voltage, generated by voltage divider 49 is coupled back to an operational amplifier, or opamp 42, which compares the feedback voltage and the reference voltage and generates an error signal at its output. The generated error signal is coupled into pass transistor 47, which is coupled between a power supply and the input TOP terminal of charge pump 15.
Some aspects of the charge pump of FIG. 4 are as follows. Pass transistor 47 is typically large and occupies valuable die area. Also, in the pumping phase the flow rate of charge from flying capacitor C1 to output capacitor Cout is not controlled. Therefore, the ripple of the output voltage is not controlled and can be quite large. Further, the loop stability restricts the selection of the external output capacitor and its equivalent series resistance. The transient performance of the charge pump is limited by the bandwidth of the control loop and can be unsatisfactory. Finally, the dynamic losses associated with turning the switch array ON and OFF can be high.